I use mats (I only have one child on them though, rest are in playpens). I like that the mats fold up for easy storage and are super easy to clean.

I like cots too and it's what we used in daycare centres. However, I just don't have the room to store when not in use.

I don't know if her staying on one or the other would help though. We used to have tons of kids play around on the cots too and try to get under them! lol